To date, microstructured materials have been obtained by rapidly solidifying composite of metal(s), alloy(s), compound(s), etc., and most of the obtained microstructured materials have particle sizes of several microns. In recent years, research has been actively conducted seeking to minimize the size of materials, i.e., from the micron to nanometer order. One of the primary characteristics of nanostructures such as nanoparticles is that a high percentage of atoms exists on the particle boundary (surface). For example, the ratio may reach 40% in 5 nm nanoparticles. Nanostructured materials may have chemical and physical characteristics that differ greatly from those of microstructured materials having the same chemical composition, and nanostructured materials often exhibit desirable characteristics.
However, it is currently difficult to obtain, for example, manganese oxides (MnOx) in a nanostructured form. Usually, the particle size of transition metal oxides prepared for commercial use is in the level of microns. The characteristics of micron-scale manganese oxides when used as an oxygen reduction catalyst have been reported. For example, No. 2000-505040 of National Publication of PCT application discloses that manganese oxide materials of different oxidation states (valencies) exhibit different catalytic activities, Mn2O3 and MnOOH which are trivalent manganese compounds have higher oxygen reduction catalytic activity than Mn3O4 and Mn5O8 which have different valencies, and the oxygen reduction potential of Mn2O3 and MnOOH is observed around −0.3 V and −1.0 V, respectively.
As a method for manufacturing nanostructured manganese dioxide (MnO2), for example, a method is known wherein an aqueous potassium permanganate (KMnO4) solution is sprayed onto an aqueous sulphuric acid solution comprising manganese sulfate (MnSO4) dissolved therein to produce a synthetic reaction, a reaction product is separated out, and the reaction product is subjected to heat treatment to obtain manganese dioxide (MnO2) (the aforementioned National Publication, page 42, FIG. 2).
An example of an oxygen reduction electrode using a manganese oxide is an air-zinc battery, wherein a mixture of micron-scale trimanganese tetraoxide (Mn3O4) and manganese dioxide (MnO2) powder is used as the oxygen reduction electrode (Japanese Unexamined Patent Publication No. H10-302808, page 8, FIG. 2).
Other publications which can be mentioned in connection with the present invention are Sasaki, Takeshi, “Preparation of metal oxide nanoparticles by laser ablation,” Laser Research Foundation Vol. 28 No. 6, June, 2000 and Journal of the Electrochemical Society, 149 (4), A504-A507 (2002).